The present invention relates generally to fibrous web materials and more specifically is concerned with a process of producing porous web materials used for making infusion packages for brewing beverages, such as tea, coffee and the like and with the web materials thus produced.
It has generally been the practice in making individual cups of tea to either place the bag of tea or the like in a cup containing boiling water, or alternatively, to place the bag in an empty cup and subsequently add the boiling water. In either event, the tea bag generally tends to inflate and float to the top of the water for a time despite the high porosity of the infusion paper utilized in making the tea bags. This inflation or "ballooning effect" is generally attributable to entrapped gases and vapors within the bag that are unable to escape due to a very thin film of water surrounding the bag. The entrapped gases, both condensible and non-condensible, tend to build up a positive pressure within the bag, frequently causing opening of the seams of mechanically sealed bags, thus undesirably discharging the tea leaves into the brew and defeating the purpose of using the bag. In the mechanically sealed tea bags, the edges of the web material are brought together, folded a number of times, and the multiple fold is crimped to provide a mechanical seam securing the two edges of the web material. The mechanically sealed bags are to be distinguished from heat sealed bags where a heat seal material, usually present within the web, is subject to heat and pressure to form a heat sealed seam.
Although seam failure and leakage of tea leaves has been evidenced for some time, it has been noticed that the incidence of failure was higher when certain strength imparting binder systems were used in the infusion web material. For example, increased seam failure has been noted with binder systems using carboxy methyl cellulose (CMC) and a reaction product of epichlorohydrin and a polyamide sold under the trade name Kymene as compared with the viscose (cellulose xanthate) bonding system widely used prior thereto. Changing the binder to latex binder materials that impart comparable tensile strength to the web material, such as ethyl vinyl acetate, cross linked polyvinyl alcohol or polyvinyl chloride, appear to provide no improvement in seam integrity and may even result in significantly higher instances of seam failure and leakage.
The treatment of infusion web material in discontinuous areas with water repellent material is disclosed by Noiset et al in U.S. Pat. No. 3,386,834 as one way of minimizing the ballooning effect. The repellent treated areas are isolated spots covered from 0.1 to 40 percent of the surface of the infuser material. Additionally, Elston et al in U.S. Pat. No. 4,289,580 achieved improved infusion in heat seal tea bags when using synthetic pulp as the heat seal phase, provided that the heat seal phase is disrupted over 10 to 75 percent of the total exposed surface area. That patent notes the disadvantages with respect to infusion properties when the hydrophobic synthetic pulp phase is not disrupted. The permeability of the web material is reduced together with its wettability, and the infusion is substantially retarded or inhibited.